Pyrogenic titanium dioxide

ABSTRACT

Shaped materials useful as catalyst for preparing cyclic lactams by reacting aminocarbonitriles with water in the liquid phase in a fixed bed reactor and which have no soluble constituents under the reaction conditions, comprising pyrogenic titanium dioxide as essential constituent, these compositions being obtainable by shaping the pyrogenic titanium dioxide into shaped articles and, before or after the shaping, treating the pyrogenic titanium dioxide with from 0.1 to 30% by weight, based on the pyrogenic titanium dioxide, of an acid in which pyrogenic titanium dioxide is sparingly soluble.

SPECIFICATION

The present invention relates to shaped materials useful as catalyst forpreparing cyclic lactams by reacting aminocarbonitriles with water, saidshaped materials essentially comprising titanium dioxide.

DE-B 25 54 198 discloses shaped titanium dioxide articles obtained byshaping titanium dioxide and calcining the shaped articles at 300 to800° C., the titanium dioxide being prepared by hydrolysis of a titaniumsalt and being treated, before or after said shaping, with from 0.01 to50% by weight, based on titanium dioxide, of a mineral acid or of anorganic acid.

However, such shaped articles have the disadvantage that titaniumdioxide prepared by hydrolysis is insufficiently pure for catalyticpurposes. This leads to losses in yield and selectivity in reactionswhere such shaped articles are used as catalyst.

DE-C 32 17 751 discloses moldings useful as catalyst which are up to 99%by weight pyrogenic titanium dioxide, have an SiO₂ content of from 0 to1% by weight, accessible pore volume of 45-55% of the volume of themolding and a breaking strength of not less than 1.630 N. Such moldingshave the disadvantage that their preparation requires the use of amolding aid, sieving of the mixture and conversion of the sieved mixtureinto a flowable powder in order that tablets may be produced with theaid of a tableting press. Pyrogenic titanium dixoide is produced by hightemperature hydrolysis of a vaporizable titanium compound, normallytitanium tetrachloride, in a detonating gas (mixture of hydrogen andoxygen gas) flame.

It is an object of the present invention to provide shaped materialscomprising titanium dioxide as essential constituent which are useful ascatalyst, which do not have the disadvantages mentioned and which can beproduced in a technically simple and economical manner.

We have found that this object is achieved by shaped materials which areuseful as catalyst and which have no soluble constituents under thereaction conditions, comprising pyrogenic titanium dioxide as essentialconstituent, said shaped materials being obtainable by shaping thepyrogenic titanium dioxide into shaped articles and, before or aftersaid shaping, treating the pyrogenic titanium dioxide with from 0.1 to30% by weight, based on the pyrogenic titanium dioxide, of an acid inwhich pyrogenic titanium dioxide is sparingly soluble.

The pyrogenic titanium dioxide can be present in various modificationssuch as amorphous, as anatase or as rutile or phase mixtures thereof.

The aforementioned titanium dioxide can be doped with, or comprise,compounds of main groups 1 to 7, especially 2, 3 or 4, of the periodictable, alumina, such as alpha- or gamma-alumina, or tin oxide, oftransition groups 1 to 7 of the periodic table, of the elements of theiron group or of the lanthanides, preferably cerium oxide, or actinidesand also mixtures thereof.

If desired, these catalysts may comprise up to 50% by weight in eachcase of copper, tin, zinc, manganese, iron, cobalt, nickel, ruthenium,palladium, platinum, silver or rhodium.

These catalytically active oxides are preparable in a conventionalmanner, for example by hydrolysis of the corresponding organics,alkoxides, salts with organic or inorganic acids and subsequent heatingor calcining and also pyrogenically and are generally commerciallyavailable.

According to the invention, the oxides are treated with an acid beforeor after shaping. Suitable acids include organic acids such as oxalicacid, propionic acid, butyric acid, maleic acid or inorganic acids suchas isopolyacids, heteropolyacids, sulfuric acid or hydrochloric acid.Particularly suitable catalysts are obtainable by treatment with aceticacid, formic acid, nitric acid, especially phosphoric acid.

It is also possible to use mixtures of acids.

The treatment can be carried out continuously or batchwise in one ormore stages. The individual stages can be carried out with the sameacid, different acids or identical or different mixtures of acids.

Similarly, the oxides can be treated with an acid in the form mentionedbefore and after shaping.

Preferably, the oxides are treated with an acid before shaping.

The amount of acid used according to the invention is from 0.1 to 30%,preferably from 0.1 to 10%, especially from 0.1 to 5%, by weight,reckoned as pure acid, based on pyrogenic titanium dioxide. The acid canbe mixed with a liquid diluent, such as water.

The catalysts can be prepared from the oxides without additives. It issimilarly possible to add additives such as binders, for exampletitanium dioxide sols, salts of the oxides used, soluble titanium saltcompounds, hydrolyzable titanium compounds such as titanium alkoxides oraluminum salts, such as pore-formers, for example methylcellulose,carbon fibers, fibers of organic polymers, melamine, starch powder,preferably before shaping.

The shaped articles can be present in various forms, for example asball, tablet, cylinder, hollow cylinder, pellet, granule or strand. Suchshaped articles are preparable in a conventional manner usingappropriate shaping machines such as tableting machines, extruders,rotary granulators, pelletizers or combinations thereof.

The shaped material, if desired after an acid treatment, isadvantageously dried, especially at from 20 to 120° C., preferably in aninert gas atmosphere or in the air, and then calcined, especially atfrom 400-750° C., preferably in an inert gas atmosphere or in the air.

The shaped materials can advantageously be used as catalyst forpreparing cyclic lactams by reacting aminocarbonitriles with water inthe liquid phase in a fixed bed reactor.

To this end, the heterogeneous catalysts can be arranged in a fixed bed.The reaction can take place in a conventional manner, for example in adownflow or preferably upflow mode, especially continuously, by bringingthe reaction mixture into contact with the catalyst bed.

The starting materials used in the process of the present invention areaminocarbonitriles, preferably those of the general formula I

where n and m are each 0, 1, 2, 3, 4, 5, 6, 7, 8 or 9 and n+m totals atleast 3, preferably at least 4.

R¹ and R² can in principle be substituents of any type. It is merelynecessary to ensure that the desired cyclization reaction is notaffected by the substituents. Preferably, R¹ and R² are independentlyC₁-C₆-alkyl or C₅-C₇-cycloalkyl or C₆-C₁₂-aryl.

Particularly preferred starting compounds are aminocarbonitriles of thegeneral formula

H₂N—(CH₂)_(m)—C≡N

where m is 3, 4, 5 or 6, especially 5. When m=5, the starting compoundis 6-aminocapronitrile.

In the process of the present invention, the above-describedaminocarbonitriles can be reacted with water in the liquid phase usingheterogeneous catalysts to form cyclic lactams. Use ofaminocarbonitriles of the formula I results in the corresponding cycliclactams of the formula II

where n, m, R¹ and R² are each as defined above. Particularly preferredlactams are those where n is 0 and m is 4, 5 or 6, especially 5(caprolactam being obtained in the latter case).

The reaction can be carried out in the liquid phase at generally from140 to 320° C., preferably at from 160 to 280° C.; the pressure isgenerally within the range from 1 to 250 bar, preferably from 5 to 150bar, it being necessary to ensure that the reaction mixture ispredominantly liquid under the conditions employed. The residence timesare generally within the range from 1 to 120, preferably from 1 to 90,and especially from 1 to 60, min. In some cases, residence times of from1 to 10 min have proven to be completely adequate.

The amount of water used per mole of aminocarbonitrile is generally atleast 0.01 mol, preferably within the range from 0.1 to 20 mol,especially within the range from 1 to 5 mol.

The aminocarbonitrile can be advantageously used in the form of a from 1to 50% strength by weight, especially from 5 to 50% strength by weight,particularly preferably from 5 to 30% strength by weight, solution inwater (in which case the solvent is also reactant) or in water/solventmixtures. Examples of usable solvents are alkanols such as methanol,ethanol, n- and i-propanol, n-, i- and t-butanol and polyols such asdiethylene glycol and tetraethylene glycol, hydrocarbons such aspetroleum ether, benzene, toluene, xylene, lactams such as pyrrolidoneor caprolactam, or alkyl-substituted lactams such asN-methyl-pyrrolidone, N-methylcaprolactam or N-ethylcaprolactam, andalso carboxylic esters, preferably of carboxylic acids having from 1 to8 carbon atoms. Ammonia can also be present in the reaction. Mixtures oforganic solvents can also be used. Mixtures of water and alkanols in awater/alkanol weight ratio of 1-75/25-99, preferably 1-50/50-99, havebeen found to be particularly advantageous in some cases.

It is in principle equally possible to use the aminocarbonitriles assolvent as well as reactant.

The advantage of the process of the present invention lies in the optionto operate the cyclization continuously in a simple manner with veryhigh throughputs and high yields and selectivities and short residencetimes. Since the catalysts used have a long lifetime from observationsto date, the result is an extremely low catalyst consumption.

EXAMPLE Example 1 Preparation of Pyrogenic Titanium Dioxide Extrudates

8350 g of pyrogenic titanium dioxide powder having a rutile/anataseratio of 80/20 were kneaded for 3 hours with 47 g of 85% strength formicacid and 3750 g of water and thereafter molded in the extruder into 4 mmextrudates under a molding pressure of 70 bar. The extrudates were driedat 120° C. for 16 hours and then calcined at 500° C. for 3 hours.

Analysis of extrudates:

Density 989 g/l Water regain 0.31 ml/g Cutting hardness 25 N Surfacearea 37 m²/g

Examples 2 to 7 Conversion of 6-aminocapronitrile into Caprolactam

A solution of 6-aminocapronitrile (ACN) in water and ethanol in theweight ratios reported in the table was passed into a 25 ml capacityheated tubular reactor (diameter 6 mm; length 800 mm) packed withcatalysts 1 and 2 recited in the table, in the form of granules. Theproduct stream leaving the reactor was analyzed by gas chromatography.The results are recited in the table as examples.

As well as caprolactam, the product stream comprises essentially ethylε-aminocaprylate and ε-aminocaprylamide. Both can likewise be cyclizedto form caprolactam. In addition, the stream includes from 5 to 8% ofcaprolactam oligomer which can be cracked to form caprolactam monomer.

TABLE Molar ratio Residence ACN Capro ACN Water ACN/H₂O Ethanol Temp.time conversion selectivity Ex. Catalyst [% by wt.] [% by wt.] [%] [% bywt.] [° C.] [min] [%] [%] 2 1 10 3.2 2 86.8 230 22 99 88 3 1 10 3.2 286.8 230 9 99 92 4 1 10 3.2 2 86.8 230 5 96 90 5 2 10 3.2 2 86.8 230 20100 91 6 2 10 3.2 2 86.8 230 8 96 92 7 2 10 3.2 2 86.8 230 5 87 90

Catalysts 1 and 2 were prepared similarly to catalyst example 1:

Catalyst 1: Pyrogenic titanium dioxide extruded with 3% of phosphoricacid as 4 mm extrudates and then ground to granules 1.6-2.0 mm in size

Catalyst 2: Pyrogenic titanium dioxide extruded with 0.5% of formic acidas 4 mm extrudates and then ground to granules 1.6-2.0 mm in size

We claim:
 1. Shaped materials, comprising pyrogenic titanium dioxide as essential constituent, said shaped materials being obtainable by shaping the pyrogenic titanium dioxide into shaped articles and, before or after said shaping, treating the pyrogenic titanium dioxide with from 0.1 to 30% by weight, based on the pyrogenic titanium dioxide, of nitric acid, acetic acid or formic acid.
 2. Shaped materials as claimed in claim 1, further comprising aluminum oxide, tin oxide, cerium oxide or mixtures thereof. 